Teach Git to write a commit graph file by checking all packed objects
to see if they are commits, then store the file in the given pack
directory.

I'm afraid that scanning all packed objects is a bit of a roundabout
way to approach this.

In my git repo, with 9 pack files at the moment, i.e. not that big a
repo and not that many pack files:

  $ time ./git commit-graph --write --update-head
  4df41a3d1cc408b7ad34bea87b51ec4ccbf4b803

  real    0m27.550s
  user    0m27.113s
  sys     0m0.376s

In comparison, performing a good old revision walk to gather all the
info that is written into the graph file:

  $ time git log --all --topo-order --format='%H %T %P %cd' |wc -l
  52954

  real    0m0.903s
  user    0m0.972s
  sys     0m0.058s

+char* get_commit_graph_filename_hash(const char *pack_dir,
+				     struct object_id *hash)
+{
+	size_t len;
+	struct strbuf head_path = STRBUF_INIT;
+	strbuf_addstr(&head_path, pack_dir);
+	strbuf_addstr(&head_path, "/graph-");
+	strbuf_addstr(&head_path, oid_to_hex(hash));
+	strbuf_addstr(&head_path, ".graph");

Nit: this is assembling the path of a graph file, not that of a
graph-head, so the strbuf should be renamed accordingly.

+
+	return strbuf_detach(&head_path, &len);
+}